If you’ve ever experienced a thunderstorm, you’re well familiar with the ability of Earth to build a static charge on its surface. When that static build-up reconnects with a similar build-up in the sky, the resulting current is seen as lightning. We’ve long known that a similar static buildup can occur on other solar system bodies. We’ve observed lightning storms on Jupiter, Saturn and Venus, for example. Of course these planets all have thick atmospheres, so what about bodies without atmospheres?

One example we know of is the Moon. Data from the Lunar Prospector mission found that the portions of the Moon’s surface could build electrostatic potentials as high as 4,50o volts. They are generated either when the Moon passes through Earth’s magnetotail, or when a solar storm bombards the Moon with charged particles. With no atmosphere the Moon can’t discharge these as lightning, so it generally leaves the surface gradually. Sometimes static charge can build within the dust of the lunar surface. The charged dust particles repel each other, and this can create levitated dust clouds. Such an effect was seen during the Apollo missions.

It has generally been thought that charge could build on the surface of other airless bodies, but there hasn’t been any direct evidence of it. Now a new paper confirms the effect for Saturn’s moon Hyperion. The authors looked at data from the Cassini mission, specifically a detector known as the Cassini Plasma Spectrometer (CAPS). This device looks at the energy of charged particles striking Cassini. During a close approach of Hyperion, CAPS detected a strong current of electrons. It was a discharge of about 200 volts over a distance of 2,000 kilometers.

Hyperion doesn’t interact strongly with Saturn’s magnetosphere, so it’s thought that the moon’s charge build-up is due to ultraviolet light striking its surface, which can knock electrons away from the surface via the photoelectric effect. This supports the idea that other outer planet moons can experience similar charges on their surface.

Just as we can get a charge out of seeing our spacecraft make a close approach of a moon, it seems the spacecraft itself can also get a charge.

For an inner planet, Earth is bountiful with water. The origin of that water has been a matter of some debate. One idea is that a combination of Earth’s strong magnetic field and distance from the Sun allowed Earth to retain much of the water emitted from rocks as the planet cooled. Another is that water came to Earth through cometary or asteroid bombardment. But now it seems the origin of Earth’s water is more complex and more interesting that we’ve thought.

Last month an article in Science showed that much of Earth’s water existed before the formation of the solar system. The authors demonstrated this by looking a levels of deuterium in terrestrial water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. As a result, it’s almost twice as heavy as regular hydrogen, and this means the way it chemically reacts is slightly different from regular hydrogen.

Deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million. When the team measured levels of deuterium in the water of Earth and other solar system bodies, they found the water contained deuterium at about 150 parts per million. This is interesting, because deuterium water is more likely to form in interstellar space. Water formed in the heat of a young solar system isn’t likely to produce much deuterium water. Given measured deuterium levels, the authors calculate that about half of Earth’s water was produced in the depths of space, before the solar system was formed.

This month another paper in Science found that water arrived on Earth earlier than expected. In this paper the team compared chondrite minerals on Earth with chondrite asteroids, specifically ones that likely originated from Vesta. Chondrite asteroids have a high quantity of water chemically bound to them, and one idea is that they could have been the source of Earth’s water. When they looked at the chemical makeup of terrestrial chondrites, they found them to be remarkably similar. This likely means terrestrial chondrites were themselves the source of Earth’s water. If that’s the case, then Earth was likely a water world a hundred million years earlier than the bombardment model predicts.

So it seems that Earth’s seas are more ancient both in origin and composition than we once thought.